This invention relates to a new process for the purification of organic isocyanates or isocyanate mixtures by removing chlorine compounds. In this process, the isocyanate or isocyanate mixture is mixed with a gel-type or macroporous, anion-exchanging organic material having tertiary and/or quaternary amino groups. The process renders possible a purification of the isocyanates from chlorine-containing compounds under milder conditions than those used in conventional purification processes and is therefore particularly suitable for treating temperature-sensitive isocyanates.
Impurities of varying type and quantity which arise during production of isocyanates are a cause of fluctuation in activity of the isocyanate. Such fluctuations in activity are a disadvantage because reproducible results can not be obtained. Both aromatic isocyanates (for example, the well-known phosgenation products of aniline-formaldehyde condensates and 2,4-and 2,6-toluenediisocyanate) and aliphatic isocyanates (such as isophorone diisocyanate) contain an abundance of such impurities. These impurities include chlorine-containing compounds which invariably cause fluctuations in activity when "freely mobile" (so-called "hydrolyzable") chlorine is involved. A proportion of these chlorine-containing compounds proves to be relatively stable and remains in the isocyanate(s) even after distillation. This proportion adversely affects the stability of the isocyanates, as well as their activity. A uniform, low proportion of these impurities with a resulting standardization of the activity and easier processing of the isocyanates is therefore important both technically and economically.
There have accordingly been many attempts to find possible ways to remove the chlorine-containing compounds. Thermal processes are described in a multitude of published patent applications. It is known, for example, that heating of isocyanates, in particular with simultaneous stripping with inert gas, or heating in inert solvents under pressure with simultaneous removal of the volatile compounds by filtration under suction, decreases the content of readily decomposable chlorine compounds. (See, for example, DE-A 1,270,036; DD 271,820; U.S. Pat. No. 3,219,678; GB-A 1,080,717; DE-A 2,237,552; U.S. Pat. No. 3,857,871; U.S. Pat. No. 1,458,223; JP 0 727 808 8 A2; JP 0 634 570 7 A2; and GB-A 1,384,065.)
JP 6 116 125 A; JP 0 516 323 1 A; DE-A 1,950,101; DE-A 1,938,384; DE-A 2,532,722; DE-A 2,631,168; U.S. Pat. No. 3,853,936; FR-A 1,555,517; DE-A 2,933,601; and U.S. Pat No. 3,549,504 each disclose that isocyanates can be purified by specific distillation and crystallization techniques. However, efficient separation of the interfering chlorine compounds is not achieved by any of these processes which are based on a purely physical treatment. Only readily decomposable chlorine compounds can be separated by these treatments. The usefulness of processes of this type is therefore limited to specific, generally thermostable, isocyanate compounds that can be used even with limited lowering of the chlorine content.
In addition to purely thermal treatments of isocyanate compounds, treatments with additives which allow an improved separation of interfering chlorine compounds are also described in the prior art. JP 4 501 032 9 B; JP4 2,004,137 B; JP59,088,452A; JP5,910,875 3A;JP 59,172,450A; U.S. Pat. No. 3,373,182; GB-A 1,111,581; U.S. Pat. No. 3,759,971; U.S. Pat. No. 4,094,894; ZA-A 8,100,606; DE-A 1,138,040; DE-A 1,286025; U.S. Pat. No. 3,458,558; U.S. Pat. No. 3,264,336; JP 01,052,747 A2; SU 806,677; and DE-A 2,210,607 disclose processes using additives based on metals or on alkali metals, for example, metal oxides, metal cyanamides, metal hydrides, metal fatty acid esters in the presence of sterically hindered phenols, metal naphthenates, metal silicates, alkali metal carbonates, organometallic compounds or (alkali) metal-containing synthetic zeolites. But in some cases these additives cannot be satisfactorily separated from the purified isocyanate and lead to unwanted metal/metal ion contamination of the corresponding isocyanate products. In addition, almost all metals and metal complexes give rise to an increased formation of secondary products (formation of trimers, carbodiimides, dimers).
Similar limitations are found with the use of additives such as the imidazole described in GB-A 1,347,647 and JP 0 505 898 2 A; the sulfonic acids and their esters described in GB-A 1,458,747; the diethyl sulfate described in GB-A 1,459,691 and the sulfuric acid also described in GB-A 1,459,691; as well as with the use of other additives such as, for example, epoxy compounds (DE-A 2,249,375; JP 0 932 396 8 A2), tetra-substituted ureas (DD 288 598), formic acid or acetic acid or their derivatives (U.S. Pat. No. 3,799,963) or the compounds containing trimethylsilyl groups described in EP-A 524 507. The described use of dissolved acids and bases, especially in the purification of reactive isocyanate compounds, leads to unwanted secondary reactions such as trimerization, dimer formation or carbodiimide formation.
Several compounds having at least one Zerewitinoff-active NH group, such as ureas (DD 285 594), biurets (DD 288 597), caprolactam (DD 285 593), epoxides in the presence of amines (JP 0 932 396 8 A2), as well as ammonium salts (DD 288 594), carbodiimides (DD 288 599), alkyl phosphates (DD 288 596), tertiary alcohols and tertiary alkyl carbamates (DD 288 595) are recommended in prior art for the purification of isocyanates. Here, too, there is the disadvantage of incomplete separation of the additives and limited use of the additive-containing isocyanates and/or distillation residues. In particular, the presence of additives may sometimes cause a significant decrease in the NCO value and an increase in viscosity, which can be attributed, for example, to the formation of biurets when tertiary alcohols are used. Increased viscosity due to formation of unwanted by-product formation also occurs when water is used to purify isocyanates (DE-A 1,240,849). U.S. Pat. No. 4,996,351 describes the use of polymeric, strongly acidic material, having pK.sub.a values of .ltoreq.2 to lower the quantity of hydrolyzable chlorine. In the patent examples, the polymeric, strongly acidic material is added to the boiling isocyanate to effectively lower the content of hydrolyzable chlorine. Inorganic acids and strongly acidic organic compounds are described as effective catalysts for allophanate formation (for example, U.S. Pat. No. 4,160,080). However, such acidic compounds greatly promote corrosion and are not therefore very suitable for decreasing the hydrolyzable chlorine content of organic isocyanates.
Patent DD 288 593 describes the use of salts of primary and secondary amines as additives for lowering the content of hydrolyzable chlorine. At temperatures of at least 200.degree. C. and with the simultaneous addition of inert gases, these salts allow an effective lowering of the content of hydrolyzable chlorine. In the examples, temperatures of 225.degree. C. are said to be necessary for an effective lowering of hydrolyzable chlorine. These high temperatures limit the application of the process to a few relatively thermostable isocyanates. According to generally accepted doctrine, isocyanate compounds, in particular temperature-sensitive low-molecular isocyanates, are not stable at such high temperatures and decompose to form unwanted by-products such as carbodiimides and isocyanurates. This can lead to an uncontrolled formation of process heat, which renders management of the reaction far more difficult. Furthermore, use of inert gas which DD 288 593 teaches to be necessary inevitably leads to a highly contaminated flow of waste gas that is potentially harmful to the environment and a safety hazard. Finally, it is difficult to separate the described salts of primary and secondary amines from purified low-molecular isocyanates by means of distillation, extraction or other known techniques because, as described in DD 288 593, the unwanted reaction of amine salt with the isocyanate(s) to be purified leads to high losses in yield and formation of secondary products.